Managing wheel skid in a locomotive

ABSTRACT

The present invention is directed to the termination of the occurrence of wheel slip/skid and prediction and prevention of the onset of wheel slip/skid in a locomotive. In one configuration, a lookup table of adhesion factors is used to predict the occurrence of wheel slip/skid.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefits, under 35 U.S.C. §119(e), ofU.S. Provisional Application Ser. No. 60/545,673, filed Feb. 17, 2004,of the same title, which is incorporated herein by this reference.

Cross reference is made to U.S. Pat. No. 6,812,656 issued Nov. 2, 2004entitled “SEQUENCED PULSE WIDTH MODULATION METHOD AND APPARATUS FORCONTROLLING AND POWERING A PLURALITY OF DIRECT CURRENT MOTORS”, U.S.patent application Ser. No. 10/649,286, filed Aug. 26, 2003, entitled“METHOD FOR MONITORING AND CONTROLLING TRACTION MOTORS IN LOCOMOTIVES”,and Ser. No. 10/650,011, filed Aug. 26, 2003, entitled “METHOD FORMONITORING AND CONTROLLING LOCOMOTIVES”, each of which is incorporatedherein by this reference.

FIELD

The present invention relates generally to diesel-electric locomotivesand specifically to wheel slip and skid management for a locomotiveemploying multiple independently controllable traction motors.

BACKGROUND

Existing railroad locomotives are typically powered by a diesel enginewhich utilizes an alternator to deliver electric power to tractionmotors which in turn power the drive wheels of the locomotive. The powerto the traction motors is typically provided by a single chopper for DCtraction motors or a single inverter for AC traction motors. One of thepresent inventors has disclosed a method and apparatus for controllingpower provided to DC traction motors by furnishing an individual choppercircuit for each traction motor in U.S. Pat. No. 6,812,656 which isincorporated herein by reference. In this invention, independentlycontrollable pulse width modulated power pulses are sequentially senteach motor. This patent discloses the practice of power reduction toindividual motors to eliminate non-synchronous wheel slip.

As described in U.S. Pat. No. 6,208,097, when a locomotive accelerates,the traction motors apply torque to the driving axles which is convertedto tractive effort of the wheels on the rails. When braking, an airbrake system and often the motors themselves, may be used to apply abraking force on the rails. Maximum tractive or braking effort isachieved if each of the driving axles is rotating such that its actualtangential speed is slightly higher while accelerating or slightly lowerwhen braking than the true ground speed of the locomotive. If adhesionis reduced or lost, some or all of the driving wheels may experienceslip while accelerating or skid while braking. Wheel slip or wheel skidcan lead to accelerated wheel wear, rail damage, high mechanicalstresses in the drive components of the propulsion system, and anundesirable decrease of the desire tractive or braking effort.

Various methods of detection of wheel slip and wheel skid are known andare discussed, for example, in U.S. Pat. No. 5,610,819, U.S. Pat. No.6,208,097 and U.S. Pat. No. 6,012,011. These methods include measurementof traction motor current, traction motor rpm and the use of tachometerson the driving axles.

As noted in U.S. Pat. No. 6,012,011, when wheel-slip occurs, thetraction motors continue to develop torque further exacerbating the slipand the wheel speed must be reduced to correct this runaway condition.Typically, once wheel slip is detected, power is reduced to all thewheels, regardless of how many of the driving wheels are actuallyexperiencing slip. Several techniques have been used in an attempt tocontrol wheel-slip on railroad locomotives such as:

-   -   reducing the power output to all driving wheels when wheel-slip        is detected on any axle until the wheel-slip stops    -   applying an abrader material to the rails, such as sand, to        increase adhesion.    -   application of friction brakes on the wheels that are slipping        to slow the wheels.    -   when several locomotives are located at the front of a train and        wheel-slip is detected on the leading locomotive, it can be        stopped by reducing the power of only this locomotive.

While there is substantial prior art on detection of wheel slipconditions on individual wheels or axles, there is little prior art onmeans of controlling wheel slip by controlling individual wheels oraxles. Johnson, in U.S. Pat. No. 6,012,011, discloses a traction controlsystem for detecting and remedying wheel-slippage on individual wheelsor axles. His system monitors the speed of each of the traction motorsused to drive the wheels of a locomotive. If the speed of a particulartraction motor indicates that the wheels are slipping, power is totallyremoved from that particular traction motor. While this method is animprovement in the art, independently turning traction motors on or off,even for brief periods, can still result in significant problems. Forexample, the power removed from a particular traction motor may beredistributed to the other motors until the diesel engine/electricgenerator prime power source is able to adjust to the new load. Thispower added to the other traction motors can, in turn, lead to wheelslippage on these other drive wheels, especially if they, as is oftenthe case, are themselves near the threshold of slippage. Further, anabrupt change in power to a traction motor can have the same negativeeffects as an abrupt change in power to all the motors and may includeaccelerated wheel wear, rail damage, high mechanical stresses in thedrive components of the propulsion system, and an undesirable decreaseof tractive (or braking) effort.

Thus, there remains a need for a more precise control of individualtraction motor power for better management of synchronous andnon-synchronous wheel slip and wheel skid. A more precise control ofindividual traction motor power particularly during non-synchronouswheel slip and wheel skid can lead to strategies for better predictingand preempting wheel slip and skid and for modifying adhesioncharacteristics of the rails to inhibit the onset of conditions thatlead to wheel slip and skid.

SUMMARY

These and other needs are addressed by the various embodiments andconfigurations of the present invention which is directed generally tomethods and systems for terminating wheel slip and skid, predicting theonset of wheel slip and skid, and creating and using wheel slip andwheel skid data to inhibit or preempt the onset of wheel slip and skid.

In a first embodiment, the present invention is directed to a method forterminating wheel slip including the steps of: (a) determining that oneor more wheels in a wheel set corresponding to a first traction motor ofa plurality of traction motors is experiencing wheel slip; and (b) inresponse, incrementally reducing power to the first traction motor for aselected period of time while continuing to provide power in excess ofthe reduced power to at least one other traction motor. The reducedpower level is nonzero and continues to drive, with reduced torque, thewheel set experiencing wheel slip.

In a second embodiment, a method is provided for terminating wheel skidincluding the steps of: (a) determining that one or more wheels in awheel set corresponding to a first traction motor of a plurality oftraction motors is experiencing wheel skid; and (b) in response,incrementally increasing power to the first traction motor for aselected period of time without increasing the power level, which may bezero during braking, applied to the other traction motors.

In the above embodiments, wheel slip and skid may be determined by anynumber of techniques. For example, the occurrence of wheel slip may bedetermined by (i) detecting an abrupt decrease in the traction motorcurrent, (ii) detecting an abrupt change in the traction motor currentderivative, (iii) detecting an abrupt increase in therevolutions-per-minute (rpms) of the traction motor or axle; (iv)detecting a characteristic “wheel slip” frequency response signature inthe frequency spectrum of the current in the traction motor, and/or (v)determining when the wheel speed is greater than the true ground speedof the locomotive. The occurrence of wheel skid may be determined, forexample, by (i) detecting an abrupt decrease to zero of the armaturevoltage of an individual traction motor, (ii) detecting an abruptdecrease to zero in the revolutions-per-minute (rpms) of an individualan individual traction motor, (iii) detecting an abrupt decrease to zeroin the revolutions-per-minute (rpms) of an individual wheel or axle,(iv) detecting an abrupt increase in traction motor current or currentderivative, (v) detecting the disappearance of commutator noise in thetraction motor current, and/or (vi) determining when a wheel speed hasstopped relative to the true ground speed of the locomotive.

In a third embodiment, a method is provided for inhibiting the onset ofwheel slip and/or skid in an accelerating locomotive. For inhibiting theonset of wheel slip, the method includes the steps of: (a) receiving arequested notch setting, the requested notch setting providing morepower to a plurality of traction motors than a current notch setting;(b) in response to the receiving step (a), determining whether wheelslip is likely for one or more wheels in a wheel set if the notchsetting is implemented; and (c) when wheel slip is likely to occur,either: (i) implementing the requested notch setting but adjusting apower level associated with the requested notch setting for individualmotors to inhibit the onset of wheel slip; or (ii) ignoring therequested notch setting and maintaining the current notch setting. Forinhibiting the onset of wheel skid, the method includes the steps of (a)braking at least one wheel set; (b) in response to the braking step (a),determining that wheel skid is likely for one or more wheels in a wheelset; and (c) when wheel skid is likely to occur, implementing an actionto preempt the onset of wheel skid. Preemptive actions include applyingless air pressure to the braking system and/or operating some or all ofthe traction motors at a positive power level to independently feathercontrol of the braking force to individual wheels.

In a fourth embodiment, a lookup table of adhesion coefficients andassociated locomotive/track/environmental conditions is used to predictthe onset of wheel slip and/or skid. Adhesion coefficients can bedetermined wheel set-by-wheel set for each of wheel slip and skid. Wheelslip may be deliberately induced in a wheel set and used to generate anadhesion coefficient. In an illustrative example for wheel slip, whenwheel slip occurs, an adhesion coefficient in effect at a selected pointbefore and/or during the occurrence of wheel slip is determined. Powerpulse widths and/or amplitudes to a selected traction motor can beincrementally increased until wheel slip occurs. An adhesion coefficientassociated with wheel skid can be determined by monitoring, for example,the armature voltage, current or rpms of an individual traction motor orthe rpms of an individual wheel or axle. The wheel skid lookup table canbe used by a controller to predict the onset of wheel skid using avariable such as a pressure in the air bake system. Wheel skid may alsobe deliberately induced in a wheel set and maintained for a timesufficient to determine an adhesion coefficient. Deliberately inducingwheel slip/skid to generate additional entries to the adhesioncoefficient table can be done traction motor-by-traction motor fordiffering locomotive/rail/environmental conditions. In this manner, thedifferent properties of each traction motor/wheel set and the resultingdifferent adhesion coefficients can be taken into account. For addedinsurance against wheel slip/skid, each of the adhesion coefficients canbe appropriately adjusted by a safety factor so that the powerlevel/braking force are well below that required to cause wheelslip/skid.

In another embodiment, wheel slip is deliberately induced in a wheelset, which is generally the front wheel set, and maintained for a timesufficient to condition a rail section over which the locomotive passes.

In one configuration, the method is applied to a locomotive wheredifferent traction motors drive wheel sets having different sets ofadhesion factors. Differing sets of adhesion coefficients for individualwheel sets may arise from differences in traction motors, drive trainand wheel variances and weight shifting amongst truck assemblies knownto occur during acceleration. For each of the traction motors, a powerlevel may be adjusted around the nominal power setting for eachrequested notch setting to inhibit the onset of wheel slip in thecorresponding wheel set. The same is true for braking force to inhibitthe onset of wheel skid.

In a further configuration, a controller predicts the onset of wheelslip using a variable, such as a torque, a tractive effort, a tractionmotor current and/or a traction motor speed associated with therequested notch setting. The variable is compared with a predeterminedvariable of the same type at and/or above which wheel slip is likely tooccur. The predetermined variable is typically derived from anoperational wheel slip history. If conditions for wheel slip arepredicted, then preemptive action may be taken. Such preemptive actionsinclude some or all of operating at reduced power, applying rail sandersor progressively reducing power in small increments beginning with theleading wheel set.

The use of individual power control circuits for each drive axle affordsa straightforward means of smoothly removing and then restoring power toa selected drive axle. The flexibility of individually controlling powerto the traction motors can be an efficient and effective approach toinhibiting and correcting non-synchronous wheel slip (duringacceleration or motoring) or wheel skid (during braking) and byextension synchronous wheel slip and wheel skid; can be used todetermine the adhesion coefficient of the rails; and can be used toeffect some conditioning of the rails by causing one set of wheels topurposely slip. The various embodiments can avoid the operationalproblems associated with an immediate termination of power to thetraction motor having a wheel set experiencing wheel slip. Theseoperational problems include the immediate and concomitantredistribution of power to the other motors until the dieselengine/electric generator prime power source is able to adjust to thenew load potentially leading to wheel slip on one or more other wheelsets, wheel wear, rail damage, high mechanical stresses in the drivecomponents of the propulsion system, and an undesirable decrease oftractive (or braking) effort.

These and other advantages will be apparent from the disclosure of theinvention(s) contained herein.

The above-described embodiments and configurations are neither completenor exhaustive. As will be appreciated, other embodiments of theinvention are possible utilizing, alone or in combination, one or moreof the features set forth above or described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a prior art chopper circuit.

FIG. 2 shows an electrical schematic of a prior art battery-dominanthybrid locomotive with four DC traction motors.

FIG. 3 is a block diagram illustrating a prior art electrical controlsystem for individually controlling a plurality of direct currentmotors.

FIG. 4 shows a series of sequential but non-overlapping power controlpulses used in the prior art where all pulses have the same power.

FIGS. 5 through 10 show examples of sequencing power pulses to fourindividual motors where one of the motors is slightly increased anddecreased in power.

FIGS. 5 a, b and c show the power pulses sent to four traction motors ata chopper frequency of 250 Hz where the power pulse to each tractionmotor is 15% of its maximum possible width.

FIGS. 6 a, b and c show the power pulses to each traction motor wherethe power pulses are 30% of their maximum possible width.

FIGS. 7 a, b and c show the power pulses to each traction motor wherethe power pulses are 45% of their maximum possible width.

FIGS. 8 a, b and c show the power pulses to each traction motor wherethe power pulses are 60% of their maximum possible width.

FIGS. 9 a, b and c show the power pulses to each traction motor wherethe power pulses are 75% of their maximum possible width.

FIGS. 10 a, b and c show the power pulses to each traction motor wherethe power pulses are 90% of their maximum possible width.

FIG. 11 shows a plot of traction motor torque output versus motorcurrent.

FIG. 12 shows a plot of traction motor tractive effort versus motor rpm

FIG. 13 illustrates a current history of a traction motor illustrating awheel slip arrest procedure.

FIG. 14 shows an example of a motor torque effort versus motor currentcurve where the level of the wheel slip adhesion coefficient ismodified.

FIG. 15 shows an example of a motor tractive effort versus motor rpmcurve with a region of track adhesion coefficients in and above whichwheel slip may occur.

FIG. 16 shows an example of a motor tractive effort versus motor currentfor current approaching the region in and above which wheel slip mayoccur.

FIG. 17 shows the logic flow for wheel slip control including preemptiveaction is taken.

FIG. 18 shows an example of tractive effort versus distance along thetrack with a band of wheel slip adhesion coefficients.

FIG. 19 shows a plot of a family of traction motor tractive effortcurves versus wheel speed.

DETAILED DESCRIPTION

In the following description, the invention is illustrated primarily byreference to a locomotive with DC traction motors where a choppercircuit is associated with each DC traction motor. Each DC motor may beindependently controlled by varying the pulse width or amplitude of thechopped power pulses. It is understood that the invention may also beapplied to a locomotive with AC traction motors where an invertercircuit is associated with each AC traction motor. Each AC motor may beindependently controlled by varying the output AC frequency or amplitudeof the inverted power pulses.

All of the principal elements of the locomotive are monitored,co-ordinated and controlled by a controller such as, for example, aProgrammable Logic Circuit (“PLC”), a micro-controller, or an industrialcomputer. The controller includes a detection scaling function which islogic for determining non-optimal performance, such as wheel slip. Powerreduction to individual motors is put in place in the case ofnon-synchronous (also known as differential) wheel slip and overallpower is reduced in the case of synchronous wheel slip.

The controller and a pulse width modulation module used in the presentinvention allow for pulse widths to individual motors to be controlledindependently. The power pulses to individual traction motors are timesequenced by the controller which directs time-sequenced power pulses toindividual chopper circuits and their coresponding DC traction motors.Power to each motor is increased by increasing pulse width whilemaintaining chopper frequency constant.

The ability to individually control the pulse width applied to eachtraction motor opens up the possibilities to tailor the power to eachtraction motor which in turn allows a number of wheel slip managementtechniques that cannot be implemented by previous traction motor powersystems discussed in the body of prior art.

In normal operation, the pulse widths sent to each traction motor arethe same for each motor. To increase power to the drive axles, the pulsewidths are increased and, normally, all pulse widths are increased bythe same amount. To decrease power to the drive axles, the pulse widthsare decreased and, normally, all pulse widths are decreased by the sameamount.

With the individual chopper circuits, if required, the width, andtherefore the amount of power to an individual motor can be modifiedrelative to those delivered to the other motors to increase or decreasepower to the selected motor.

In a preferred embodiment, if the wheels on one or more of the driveaxles is determined to be slipping during acceleration, then the powerto the traction motor driving that axle experiencing wheel slip can bereduced in small, predetermined increments until the cessation of wheelslip is detected. This is an improvement over the art of U.S. Pat. No.6,012,011 in which, when wheel slip is detected on an individual driveaxle, the power is completely switched off until wheel slip isdetermined to have stopped.

In another embodiment of the invention, if the wheels on one or more ofthe drive axles are determined to be skidding during braking, then thepower to the traction motor driving that axle experiencing wheel skidcan be increased in small, predetermined increments until the cessationof wheel skid is detected. This improvement in braking control is notpossible with the method disclosed in U.S. Pat. No. 6,012,011 in whichthe power to an individual drive axle can only be completely switchedoff. In a further aspect of this invention, a small level of positivepower can be applied to the traction motors during braking to act as ameans of detection of wheel skid or lock-up. The small amount ofpositive power will require a small amount of additional braking butwill provide field current to the traction motor which can be used todetect wheel skid. Alternately, occurrence of wheel skid may bedetermined by (i) detecting an abrupt decrease to zero of the armaturevoltage of an individual traction motor (ii) detecting an abruptdecrease to zero in the revolutions-per-minute (rpms) of an individualwheel or axle, (iii) detecting the disappearance of commutator noise inthe traction motor current history, (iv) detecting an abrupt increase intraction motor current or current derivative, and/or (v) determiningwhen a wheel speed has stopped relative to the true ground speed of thelocomotive.

In a more preferred embodiment, automatic actions are taken to preemptwheel slip by monitoring any two of the traction motor current history,derivative of the traction motor current history, motor torque, motortractive effort, motor rpms and comparing these to a torque or tractiveeffort map of the motor stored in an on-board computer. The torque ortractive effort map is a compilation of the motor torque or tractiveeffort versus motor current or rpm for various rail contact friction oradhesion coefficients. Each motor may have a slightly different torqueor tractive effort map as a result of differences in motorcharacteristics, weight on the axle and/or location of the drive axle onthe locomotive. The computer also contains information on the torque ortractive effort map regions where the wheels may approach slipconditions. As the boundary to these regions is approached, the power tothe motor connected to that axle is reduced incrementally until the axletorque or tractive effort is lowered to a predetermined distance on thetorque or tractive effort map below the onset of wheel slip.

In another aspect of the preempting logic the predetermined thresholdlocations of wheel slip may be automatically modified to a lowerthreshold if wheel slip is detected to be occurring too frequently. Thiscan be accomplished by a mathematical or logical algorithm or by aself-learning logic such as embodied in a neural network computationalprocess.

In yet another aspect of the preempting logic, if the adhesioncoefficient is determined to be low or becoming low or if tractiveeffort is approaching the adhesion limit, then rail sanders can beactivated automatically to increase adhesion or traction.

In another aspect of the invention, the ability to incrementallyincrease the power to a particular traction motor enables the ability todetermine local rail adhesion conditions. By incrementally increasingpower to a particular motor, it is possible to induce wheel slip andthereby determine the adhesion coefficient of the wheels on the rails.This information can be used, for example, to help determine thethreshold for wheel slip for various locomotive, environmental and trackconditions and therefore improve the preempting logic.

In yet another aspect of the invention, the ability to incrementallymodify the power to a particular traction motor enables the ability todetermine local rail adhesion conditions during braking. For example,the brakes may be applied and at the same time a modest amount of powercan also be applied to all the traction motors. By incrementallydecreasing power to a particular motor, it is possible to induce wheelskid momentarily and thereby determine the adhesion coefficient of thewheels on the rails. This information can be used, for example, to helpdetermine the threshold for wheel skid for various locomotive,environmental and track conditions and therefore improve the preemptinglogic.

Further, the ability to incrementally modify the power from anindividual motor relative to that delivered to other motors can be usedto cause wheel slippage to help condition the rails for the remainingdrive axles. For example, the leading drive wheels can be caused to slipand condition the rails, by removing moisture, ice or the like, for thesubsequent or trailing drive wheel pairs.

The main elements of a prior art chopper circuit as used in the presentinvention, are shown in FIG. 1. The chopper circuit has input terminals1001 through which current flows into the circuit. The main current flowis along path 1004 which passes through an Insulated Gate BipolarTransistor (IGBT) switch 1003 and a traction motor 1002. The maincurrent path 1004 is active when the input power source (not shown) ispowering the traction motor 1002. When the IGBT 1003 is switched to itsoff position, current is forced to flow through the free-wheeling path1006 by the free-wheeling gate 1005, which is shown as being a diode.The chopper circuit thus controls the speed of the motor by switchingthe input voltage on and off depending on what average output power isrequired; the longer the chopper is switched on, the higher the averageoutput power. The time interval during which the chopper is switched onis known as the on-time; the interval during which the chopper isswitched off is known as the off-time. The ratio of the on-time of thepower pulse to the off-time of the power pulse is often referred to asthe-mark-to-space ratio or chopper ratio.

In the present invention, there is preferably a chopper circuitassociated with each traction motor. This is illustrated in the priorart FIG. 2 which is an electrical circuit schematic showing a DC powersource, which may be a battery pack as shown here or a diesel enginewith an alternator/rectifier or both, and four traction motors, eachmotor having an individual chopper circuit. This particular circuitconfiguration was disclosed in U.S. application Ser. No. 10/649,286. Thepower supply 2001 is connected to a traction motor system 2005 bydisconnect switches 2003 which are controlled by a locomotive computersystem. The power supply voltage is monitored by voltage sensor 2021 andthe battery pack output current is measured by current sensor 2022.

The four traction motor systems 2005 are shown here connected inparallel with the power supply 2001. Each traction motor 2006 isassociated with its own individual chopper circuit 2007. The IGBT 2020is controlled by the locomotive computer system. The main currentthrough each traction motor 2006 is monitored by a current transducer2015. FIG. 2 also shows a configuration 2012 to effect the switchingnecessary to reverse the motor direction by reversing the current flowthrough the field coils.

FIG. 3, which was first disclosed in U.S. application Ser. No.10/083,587, illustrates the general control system for coordinating thethrottle input requests with the distribution of power to the tractionmotors. A locomotive drive system comprises a DC power source 3010 suchas a battery pack and a plurality (typically 4 or 6 in number) of DCmotors 3012, 3013, 3014 and 3015. An input device 3011 provides throttleinput information by means of which the operator selects the desiredspeed or power requirement. The operator also receives feedbackinformation from the controller 3016.

A controller, such as for example a PLC 3016 receives the informationfrom the input device 3011 and sends the information to the powercontrol system (chopper circuit 3018 in the power control system), whichsubsequently individually controls a plurality of DC motors 3012.

The throttle input information is provided by an input device 3011 thatthe locomotive operator uses to request the amount of power to beapplied to the rails via the motors 3012. This is typically a throttlenotch between idle and eight positions but also could be an electronicdevice, such as an infinitely variable control or a touch screen.

A power source voltage sensing device 3021 is provided to measure thevoltage of the power source 3010. A current sensing device 3052 isprovided to measure the amount of current flowing from the power source3010. Further individual motor current sensing devices 3022 enable theamount of current flowing to each DC motor 3012, 3013, 3014 and 3015 tobe measured, allowing the information to be supplied to the controller3016.

Axle alternators 3026 are electronic devices capable of measuring therevolutions of the axle on which they are installed. This information issent to the controller 3016 to determine speed or detect situations thatrequire attention and correction, such as wheel slip and wheel skid.

The controller 3016 is programmed according to usual methods to carryout the following functions. The controller processes throttle inputrequests, power source voltage, and determines current control points tosatisfy individual traction motor power requirements. It also comprisesa derate evaluation logic function 3028, which is logic to reduce thepower demand below that requested by the operator for protection ofequipment. This could include reducing power in case equipment is atrisk of overheating or currents climb close to equipment design limits.It comprises a detection scaling function 3030, which is logic fordetermining non-optimal performance, such as wheel slip. Power reductionto individual motors is put in place in the case of differential wheelslip and overall power is reduced in the case of synchronous wheel slip.Power increase to individual motors is put in place in the case ofdifferential wheel skid and overall power is increased in the case ofsynchronous wheel skid.

A ramping function 3032 is provided, which is logic to ramp requestedthrottle level at a rate that is reasonable for the locomotive. A powerdispatch logic function 3034 is also provided, which is central logicthat evaluates any pertinent derate conditions, any wheel slip, as wellas the requested throttle level, to determine the appropriate powerlevel to be sent to the pulse width modulation module 3036 as well asany individual power reductions that may be necessary.

The chopper circuit 3018 comprises the following elements. A clock 3038comprises an integrated circuit that generates a series of pulses. Asequencer 3040 is an integrated circuit that sequences the pulses intouniform periods for purposes of the pulse width regions for each motor.A pulse width modulation module 3036 provides clipped triangularwaveforms that result in the creation of a series of pulses, which isused essentially to toggle the power switch devices on and off accordingto the pulses. The drive switches 3042 are IGBTs that are switchingdevices that are capable of sequentially pulsing the power source to thedifferent motors at a very fast rate.

In prior art applications, a single chopper circuit has been used tocontrol the speed of all of the DC traction motors. This has a number ofdisadvantages. For example, if one of the wheels is slipping(non-synchronous wheel slip), the chopper reduces power to all of themotors which risks further exacerbation of the problem.

FIG. 4 shows a time sequence of short pulses 4001 to each motor typicalof locomotive start up at a low throttle condition. This figure is priorart and was disclosed in U.S. patent application Ser. No. 10/083,587.The pulses 4001 in each sequence are shown along a time axis 4002 whichis a common time axis for each sequence. Since the voltage amplitude ofthe pulses 4001 are shown here as approximately constant, the pulseamplitudes 4003 may be considered current or power amplitudes. Althoughnot required, each motor receives a power pulse 4001 at a differenttime. FIG. 4 a represents the pulses provided to a first traction motor;FIG. 4 b to a second traction motor; FIG. 4 c to a third traction motor;and FIG. 4 d to a fourth traction motor. FIG. 4 e shows the sum of theindividual motor sequences 4004.

The method illustrated In FIG. 4 is implemented by commandingtime-sequenced power pulses to individual chopper circuit/DC tractionmotor arrays. Power to each motor is increased by increasing pulse widthwhile maintaining frequency constant. Power is drawn at different timesfor different motors by starting the power pulses to each motor at aconstant offset from neighboring pulses. The offsets are the maximumpossible. At low power settings, the power pulses to each motor are notoverlapping. As the commanded power increases, the power pulses arewidened so that eventually neighboring power pulses overlap.

It is noted that the voltage amplitude of all power pulses can beconstant. Thus at low speed, the motor current pulses are maximumamplitude. As speed increases, the motor develops a back emf whichreduces the amplitude of the current pulses. This is compensated for byincreasing pulse width.

The logic controller allows for pulse widths to individual motors to becontrolled independently. Normally, all pulse widths are increased ordecreased by the same amounts. The ability to individually control thelevel of power applied to different axles results in an efficient andeffective approach to correcting wheel slip for both non-synchronous aswell as synchronous wheel slip.

The method illustrated in FIG. 4 can be summarized as follows. The levelof power applied to different axles by applying pulse width modulationto a plurality of DC traction motors by time-sequencing the pulses toindividual motors is carried out such that the time separation betweenpulses to different motors is always maximized. Note that the pulsesreferred to are the control pulses sent out by the PLC or microprocessorto the individual chopper circuits. The pulses seen by the DC motors aresmoothed out by the capacitor filters across the motors.

Differentially Modifying Power to Individual Traction Motors

The advantages of individual chopper circuits with each traction motorare illustrated in FIGS. 5 through 10 which show examples of sequencingpower pulses to four individual motors where one of the motors isslightly increased and decreased in power relative to the power levelapplied to the other three motors.

FIG. 5 a shows power pulses of equal widths sent to four traction motorsat a chopper frequency of 250 Hz. The start time of each pulse is offsetfrom the adjacent pulse by 1 millisecond 5001. In FIG. 5 a the powerpulse sent to each traction motor is 15% of its maximum possible width.Therefore each pulse is 0.6 milliseconds in width 5002. In this example,none of the pulses overlap. In FIG. 5 b, the power to motor #2 5003 isreduced by 10% so the pulse width for motor #2 5003 is now 0.54milliseconds in width while the other 3 motors have pulse widths of 0.6milliseconds. In FIG. 5 c, the power to motor #2 5003 is increased by10% so the pulse width for motor #2 is now 0.66 milliseconds in widthwhile the other 3 motors have pulse widths of 0.6 milliseconds.

In FIG. 6 a, the power pulse to each traction motor is 30% of itsmaximum possible width. Therefore each pulse is 1.2 milliseconds inwidth and the pulses partially overlap so that the total power to all 4motors is additive for a small fraction of the time 6001. In FIG. 6 b,the power to motor #2 is reduced by 10% so the pulse width for motor #2is now 1.08 milliseconds in width while the other 3 motors have pulsewidths of 1.2 milliseconds. In FIG. 6 c, the power to motor #2 isincreased by 10% so the pulse width for motor #2 is now 1.32milliseconds in width while the other 3 motors have pulse widths of 1.2milliseconds. The effect of reducing or increasing the power to onemotor (motor #2) is clearly seen in FIGS. 6 b and 6 c.

In FIG. 7 a, the power pulse to each traction motor is 45% of itsmaximum possible width. Therefore each pulse is 1.8 milliseconds inwidth and the pulses overlap enough that the total power to all 4 motorsis almost continuously twice the power output of one motor. In FIG. 7 b,the power to motor #2 is reduced by 10% so the pulse width for motor #2is now 1.62 milliseconds in width while the other 3 motors have pulsewidths of 1.8 milliseconds. In FIG. 7 c, the power to motor #2 isincreased by 10% so the pulse width for motor #2 is now 1.98milliseconds in width while the other 3 motors have pulse widths of 1.8milliseconds.

In FIG. 8 a, the power pulse to each traction motor is increased to 60%of its maximum possible width. Therefore each pulse is 2.4 millisecondsin width and the pulses substantially overlap enough that the totalpower to all 4 motors is always greater than twice the power output ofone motor and sometimes greater than three times the output of onemotor. In FIG. 8 b, the power to motor #2 is reduced by 10% so the pulsewidth for motor #2 is now 2.16 milliseconds in width while the other 3motors have pulse widths of 2.4 milliseconds. In FIG. 8 c, the power tomotor #2 is increased by 10% so the pulse width for motor #2 is now 2.64milliseconds in width while the other 3 motors have pulse widths of 2.4milliseconds.

In FIG. 9 a, the power pulse to each traction motor is increased to 75%of its maximum possible width. Therefore each pulse is 3.0 millisecondsin width and the pulses overlap enough that the total power to all 4motors is always three times the power output of one motor. In FIG. 9 b,the power to motor #2 is reduced by 10% so the pulse width for motor #2is now 2.70 milliseconds in width while the other 3 motors have pulsewidths of 3.0 milliseconds. In FIG. 9 c, the power to motor #2 isincreased by 10% so the pulse width for motor #2 is now 3.3 millisecondsin width while the other 3 motors have pulse widths of 3.0 milliseconds.

In FIG. 10 a, the power pulse to each traction motor is increased to 90%of its maximum possible width. Therefore each pulse is 3.6 millisecondsin width and the pulses overlap enough that the total power to all 4motors is ranges between three and four times the power output of onemotor. In FIG. 10 b, the power to motor #2 is reduced by 10% so thepulse width for motor #2 is now 3.24 milliseconds in width while theother 3 motors have pulse widths of 3.6 milliseconds. In FIG. 10 c, thepower to motor #2 is increased by 10% so the pulse width for motor #2 isnow 3.96 milliseconds in width while the other 3 motors have pulsewidths of 3.6 milliseconds.

At 100% pulse widths, all motors are operating continuously. In thissituation, it is possible to reduce power to one or more motors but notto increase power to any motor since they are all operating continuouslyat their maximum possible power level.

The percentages of peak locomotive power for each condition representedby FIGS. 5 through 10 are shown in the following table. Nominal PulseWidth as a 10% Power 10% power Percentage of Reduction in Motor AllMotors at Increase Motor #2 Continuous #2 Only Equal Power Only 15%14.6% 15.0% 15.4% 30% 29.3% 30.0% 30.8% 45% 43.9% 45.0% 46.1% 60% 58.5%60.0% 61.5% 75% 73.1% 75.0% 76.9% 90% 87.8% 90.0% 92.3%

The pulse widths represented in FIGS. 5 through 10 are the pulse widthssent to the chopper boards to turn the free-wheeling diodes on and off.With the ramping functions applied to the output pulses along with thefilters across the DC power supply and across each chopper circuit, thepower waveforms sent to each traction motor are smoothed out. Therefore,the technique of incrementally reducing or increasing power pulses fromthe controller results in a smooth variation of power to the tractionmotors and hence to the axles.

Traction Motor Relationships

A traction motor and its drive axle can be characterized by motorcurrent, motor RPMs, motor torque, motor power and motor tractiveeffort. These are all related by well-known mathematical relationships.These are:Motor Torque=constant1*Motor Power/RPMsMotor Power=constant2*Motor Tractive Effort*Axle SpeedAxle Speed=constant3*Motor RPMswhich leads to:Motor Torque=constant4*Motor Tractive EffortThese relations apply when the wheels are not slipping or skidding.

In addition, another well-known relation that will be used is:Adhesion Coefficient=Tractive Effort/Weight on Wheels(expressed as a percent at which the wheels begin to slip or skid. Also,it is noted that the adhesion coefficient for slip may be different thanthe adhesion coefficient for skid) The adhesion coefficient is directlyrelated to the coefficients of friction between the wheel and the railsurface.

In a conventional diesel locomotive, the weight of the locomotive canchange by approximately 12% as the locomotive consumes fuel. The changeof weight on the driving wheels as fuel is consumed can be accounted forand the estimated adhesion coefficient can be adjusted.

Mapping Individual Traction Motor Characteristics

FIG. 11 shows a plot of traction motor torque output 11001 versus motorcurrent 11002. The torque output 11001 by the motor increases as thecurrent 11002 through the traction motor increases. Since tractiveeffort is proportional to torque, the form of the tractive effort versusmotor current is the same.

Lines of constant torque (or tractive effort) 11003 represent lines ofconstant adhesion factor (or coefficient of friction). FIG. 11 shows onesuch line of constant adhesion factor 11003. For any torque above thisline, wheel slip will occur. In the present invention, each tractionmotor may have its own unique torque versus motor current curve storedin an on-board memory. These curves may differ slightly from motor tomotor because of, for example, differences in motor windings andresistance, differences in motor back emf because of mechanicaltolerances, differences in the mechanical linkage from motor to axle anddifferences in wheel diameter due, for example, wear or manufacture.Motor current may be sensed by any number of current sensing devicessuch as, for example current-sensing resistors, Hall current sensors,current-sensing transformers, current transducers, Rogowski coils orother common current measuring devices.

FIG. 12 shows a plot of traction motor tractive effort 12001 versusmotor speed 12002. As the rpms of the motor 12002 increase, the tractiveeffort 12001 output by the motor decreases. Since torque is proportionalto tractive effort, the form of the torque versus motor rpms is thesame. Lines of constant tractive effort (or torque) 12003 representlines of constant adhesion factor (or coefficient of friction). FIG. 12shows one such line of constant adhesion factor 12003. For any tractiveeffort above this line, wheel slip will occur. In the present invention,each traction motor may have its own unique tractive effort versus motorspeed curve stored in an on-board memory. Speed may be expressed inmotor rpms or in miles per hour of the wheel along the rail. Rotaryspeed sensors include tachometers, axle alternators and the like. Theseindicate the rotational speed of the wheels or axle or traction motorarmature. These are all related in a fixed way by the gear ratio andwheel diameter of the truck assembly. For example, motor alternator RPMsare equal to axle RPMs times the gear ratio.

Wheel Slip Detection and Correction

The speed of the locomotive relative to the ground (true ground speed)may be sensed, for example, by a radar system or by a GPS system. Whenany set of wheels are slipping, their indicated wheel speed should begreater than the true ground speed of the locomotive.

Wheel slip of each axle may be detected by any number of means known tothose in the art. These include, for example, detecting an abruptcurrent or current derivative decrease in the traction motor current oran abrupt increase in the rpms of the traction motor or axle, or adifference between indicated wheel speed and true ground speed, or byany combination of these.

The more preferred means of wheel slip detection is by monitoring themotor current. This is preferred because it does not require additionalequipment on the traction motor. A rotary sensor on the traction motoror drive axle is a more direct measurement of wheel slip and ispreferred if the motor or axle has a rotary sensor already in place.

Once wheel slip is detected, the controller can take action to terminatethe wheel slip, be it synchronous or non-synchronous. For example, thecontroller can begin an immediate reduction in power to the motordriving the slipping wheels by reducing the power pulse widths inpredetermined increments until wheel slip is detected to have ceased.The increments may be expressed as a percentage of the maximum pre-slipcurrent or as a percentage of the previous pulse where the first pulseis the maximum pre-slip current. The pulse width reduction incrementsare preferably in the range of 5% to 50%, more preferable in the rangeof 10 to 35% and most preferably in the range of 10 to 20% of themaximum pre-slip current.

The period for detection and corrective action may be carried outautomatically by the controller. For a locomotive setup of 4 axles and achopper frequency of 250 Hz, power pulses are sent to each axle every 4milliseconds. In this example, the sequence of power pulses can consistof a series of pulses diminishing by 10% of the maximum pre-slip currentevery 4 milliseconds until wheel slipping ceases. However, the motion ofthe slipping wheels will be much slower because of the inertia of thewheels and drive train components requiring power reduction to be slowerto match the mechanical requirements of the drive train. Nevertheless,the power to the slipping wheels can be reduced rapidly, on amillisecond time scale if necessary.

An example of a current history of a traction motor reflecting a wheelslip arrest procedure is shown in FIG. 13. In this figure, current 13001is shown as a function of time 13002. Initially, the current is slowlydecreasing 13003 as would be the case for acceleration of thelocomotive. Just before the onset of wheel slip, the wheels often makeand break contact with the rails and this manifests itself as a phase orperiod in the current history having a characteristic signature 13004.This characteristic signature can be detected by, for example, samplingthe frequency spectrum of the current history and discerning acharacteristic frequency response that indicates incipient wheel slipwhich is sometimes referred to as creep in the adhesion curve. At sometime, the wheels slip and the motor rpms increase rapidly causing agreater back emf which, in turn, results in an abrupt reduction 13005 incurrent. In the present invention, the controller reacts to this byreducing the current to the motor until the wheels stop slipping. As thewheel rpms slow down, the current slowly increases 13006 until tractionis reestablished 13007 and the current to the motor returns to a value13008 that is consistent with non-slipping motor torque. That is, thetorque and current return to their desired values as determined by thetorque versus current curve such as shown in FIG. 11.

The motor torque (or tractive effort) when the current 13004 is justbeginning to ripple indicates the adhesion coefficient for the onset ofwheel slip. This value, which may be adjusted to include an added safetyfactor, may be used to adjust the adhesion coefficient where wheel slipmay be expected to recur.

Since the tractive effort or torque of each axle is known as a functionof motor current and these curves can be stored in an on-board computer,each wheel slip occurrence can be used to give an estimate of adhesioncoefficient for that axle/wheel set and that track location. In thisway, a database of wheel slip conditions can be built up and stored forfuture use.

An example of such a curve is shown in FIG. 14. FIG. 14 shows motortorque 14001 versus motor current 14002. At the beginning of anoperation, the adhesion limit 14003 for wheel slip is shown. If wheelslip occurs prior to the limit 14003, then a new torque or tractiveeffort limit 14004 is determined from the current monitoring device suchas depicted in FIG. 13. If wheel slip continues to recur, then theadhesion limit can be further reduced to a new value 14005.

Deliberately Inducing Wheel Slip to Determine Adhesion

The ability to slightly increase or reduce power to individual axles canbe used to induce wheel slip for purposes of establishing an adhesioncoefficient. At the desired time, the controller can increase power to aselected motor by increasing the power pulse widths in predeterminedincrements until wheel slip is detected to have occurred. The incrementsmay be expressed as a percentage of the maximum pre-slip current or as apercentage of the previous pulse where the first pulse is the maximumpre-slip current. The pulse width increase increments are preferably inthe range of 1% to 25%, more preferably in the range of 1 to 15% andmost preferably in the range of 1 to 5% of the maximum pre-slip current.Once wheel slip is detected, the adhesion coefficient is recorded andwheel slip is terminated by returning the current to the pre-wheel sliplevel. If the wheels continue to slip, then the wheel slip control logicdescribed above is automatically activated until wheel slip isterminated. This process can be used to update the adhesion limits suchas shown in FIG. 14.

Deliberately Inducing Wheel Slip to Condition Rail Surface

The ability to slightly increase or reduce power to individual axles canbe used to induce wheel slip for purposes of conditioning the rails. Forexample, if the rails are oily or wet or corroded, preferably theleading set of wheels or less preferably any other set of wheels, can bemade to slip in a controllable manner so as to reduce or remove, oil,water, ice or corrosion from the rails to increase the adhesioncoefficient of the rails for the trailing wheel sets. At the desiredtime, the controller can increase power to a selected motor byincreasing the power pulse widths in predetermined increments untilwheel slip is detected to have occurred. The increments may be expressedas a percentage of the maximum pre-slip current or as a percentage ofthe previous pulse where the first pulse is the maximum pre-slipcurrent. The pulse width increase increments are preferably in the rangeof 5% to 35%, more preferably in the range of 10 to 25% and mostpreferably in the range of 10 to 15% of the maximum pre-slip current.Once wheel slip is detected, the wheels may be allowed to slip for apredetermined time so as to increase the adhesion coefficient of thetrack. Wheel slip is terminated by returning the current to thepre-wheel slip level. If the wheels continue to slip, then the wheelslip control logic described above automatically activates until wheelslip is terminated. Again, the adhesion coefficient can be recorded andadded to the data base stored in the on-board computer memory.

Logic for Preempting Wheel Slip

The ability to slightly increase or reduce power to individual axles canbe used as the basis for a strategy of minimizing the occurrence of, orpreempting wheel slip. The strategy includes one or more computer-storedmotor torque versus motor current or motor rpm curves; or a tractiveeffort versus motor current or motor rpm curve characteristic of eachdriving axle. These curves, once generated, are relatively stable andunchanging over time. From the data base of wheel slip history and knowntrack adhesion coefficients, a band can be constructed on these curves,that represents the region where wheel slip has occurred in the past. Anexample of such a curve was shown in FIG. 14 which shows motor torque14001 versus motor current 14002. The region between the maximum andminimum adhesion curves 14003 and 14005 can be considered as a band orregion where the onset of wheel slip is known to occur.

If wheel slip continues to occur, the adhesion limit curve 14005 can befurther lowered. Conversely, if wheel slip does not recur for asubstantial time, the controller can induce wheel slip such as describedabove and can determine that the adhesion coefficient can be movedupward (higher torque value) on the torque versus current curve.

The wheel slip onset regions can be varied for different track locationsand different conditions on the tracks and stored in the memory of anon-board computer for future reference.

FIG. 15 shows an example of a motor tractive effort 15001 versus motorrpm 15002 curve with a region 15003 of track adhesion coefficients inand above which wheel slip may occur. This region may be established foreach traction motor/axle combination and may be generated by pastexperience, past knowledge of a particular section of track or byinducing wheel slip to establish adhesion coefficients.

The range of tractive effort defined by the range of adhesioncoefficients illustrated in FIG. 15 can be shown as a correspondingrange on the plot of tractive effort versus motor current such as shownin FIG. 16 which shows motor tractive effort 16001 versus motor current16002 for current approaching the region 16003 in and above which wheelslip may occur. As the tractive effort is increased 16004, thecontroller monitors the approach of tractive effort to the region 16003of known wheel slip occurrence and then ensures that the rate ofapplication of power (or tractive effort) to that drive axle is slowedby a predetermined algorithm as the adhesion limit is approached. Ifwheel slip is detected, the level of tractive effort at which it occursis recorded and the wheel slip control logic described aboveautomatically activates until wheel slip is terminated. The level of thewheel slip adhesion coefficient is then lowered to reflect new wheelslip conditions and the adhesion region is appropriately updated.

An example of programmable and automated logic for wheel slip controlincluding preemptive action is shown in FIG. 17. When power is applied,each traction motor is examined in turn. For each motor:

-   -   motor current is measured    -   the motor current spectrum is analyzed    -   if wheel slip is determined to be incipient        -   power is reduced by a predetermined amount        -   the motor current spectrum is analyzed        -   if wheel slip is still incipient, reduce power again until            no longer incipient    -   once wheel slip has ceased, update the adhesion data base    -   determine if additional adhesion data is required    -   if additional data is required, induce wheel slip    -   end of cycle    -   if wheel slip is determined to be occurring        -   power is reduced by a predetermined amount        -   the motor current spectrum is analyzed        -   if wheel slip is still occurring, reduce power again until            no longer occurring    -   once wheel slip has ceased, update the adhesion data base    -   determine if additional adhesion data is required    -   if additional data is required, induce wheel slip    -   end of cycle    -   if wheel slip is determined not to be incipient nor occurring    -   determine if adhesion limit is being approached    -   if limit is being approached, reduce power to a predetermined        limit    -   determine if additional adhesion data is required    -   if additional data is required, induce wheel slip    -   end of cycle

The foregoing example is one of numerous variants of logic to manage andpreempt wheel slip. This level of wheel slip management and preemptionis only possible if the power to each individual traction motor can beslightly increased or decreased independently, as is possible in thepresent invention.

FIG. 18 illustrates an example of tractive effort 18001 versus distance18002 map of wheel slip adhesion coefficients. Such a map can bedeveloped by saving data from wheel slip occurrences, known data anddata generated by inducing wheel slip such as described above as part ofthe present invention. FIG. 18 shows two tractive effort adhesioncurves. The higher curve 18003 represents tractive effort above whichwheel slip always occurs. The lower curve 18004 represents tractiveeffort above which wheel slip or the onset of wheel slip may occur. Thismap can be used as part of the preemptive wheel slip management strategydescribed in FIG. 17.

In yet another aspect of the preempting logic (not shown in FIG. 18), ifthe adhesion coefficient is determined to be low or becoming low or iftractive effort is approaching the adhesion limit, then rail sanders canbe activated automatically to increase adhesion or traction, therebypreempting or at least further forestalling wheel slip.

Braking

If the wheels on one or more of the drive axles is determined to beskidding during braking, then the power to the traction motor drivingthat axle experiencing wheel skid can be increased in small,predetermined increments until the cessation of wheel skid is detected.Power is incrementally increased to individual motors in the case ofdifferential wheel skid and power to all the drive axles isincrementally increased in the case of synchronous wheel skid. Thisimprovement in braking control is not possible with the method disclosedin U.S. Pat. No. 6,012,011 in which the power to an individual driveaxle can only be completely switched off.

It is also possible to apply a small voltage to all motors duringbraking at low speed (typically less than 15 mph) such that the appliedvoltage is approximately the same as the back emf on the tractionmotors. If a wheel or wheels skids, then the back emf will drop to zeroand the small applied voltage will drive a substantial current throughthe motors and produce a high torque that will act to unlock theskidding wheel or wheels. If one of more wheels do not unlock, then theapplied voltage (and hence power) can be increased on the locked wheelsto further increase the torque which tends to unlock the wheels. It isunderstood that the applied voltage would automatically be maintained atapproximately the same as the back emf on the traction motors as thelocomotive speed decreases during braking. The preferred method ofmonitoring the applied voltage is to monitor the traction motor currentalthough the voltage across the motor could also be monitored. When thelocomotive comes to a complete stop, the applied voltage is turned offso that the locomotive will not tend to accelerate once the brakes arereleased.

The methods and concepts discussed above for control of wheel slip canbe applied to wheel skid during braking. In addition, by monitoringmotor current such as shown in FIG. 13 above and/or monitoring armaturevoltage and axle rpms, the detection of wheel skid can be utilized toupdate adhesion coefficients. In order to monitor motor current duringbraking, a small level of positive power can be applied to the tractionmotors during braking to act as a means of detection of wheel skid orlock-up. The small amount of positive power will require a small amountof additional braking but will provide field current to the tractionmotor which can be used to detect wheel skid. It is also possible todetect the onset of wheel skid by monitoring the current in a tractionmotor. When a wheel begins to skid, the traction motor armature ceasesto rotate, there is an abrupt rise in motor current and the commutatornoise disappears from the current trace. These behaviors can be detectedand used to determine the onset of wheel skid. The ability to controlwheel skid on individual wheel sets is of benefit especially for quicklyreacting to the onset of skid and thereby minimizing or preventing thedevelopment of flat spots on the skidding wheels.

An adhesion coefficient appropriate to wheel skid can be determined byinducing wheel skid for a brief period (a period brief enough to preventany wheel flattening). This can be done by applying a small amount ofpower to all traction motors during braking and then reducing power to aselected traction motor until a wheel or wheels on its correspondingwheel set begins to skid. The power can then be immediately restored toits pre-skid level.

FIG. 19 shows a plot of a set of traction motor tractive effort curvesversus wheel or axle speed (which is directly related to wheel rpm ortraction motor rpm). Most locomotives operate using a set ofapproximately constant power curves commonly called notch settings. Formotoring, there are usually eight power or notch settings that may beselected by the locomotive engineer. When motor current exceeds apredetermined limit, the power may be limited so that a portion of acurve may not represent constant power. FIG. 19 shows tractive effort1901 versus wheel speed 1902 for a series of approximately constantpower curves. For example, curve 1903 is the highest power setting(notch 8) and illustrates a current limit 1906 at low speeds. Curve 1904is a lower power curve and is notch 7. Curve 1905 is the lowest powercurve and is notch 1. An adhesion coefficient band represents the regionbelow whose lower boundary 1908 there is no wheel slip and above whoseupper boundary 1907 there is always wheel slip. As can be seen, each ofthe eight power curves in this example passes through the adhesioncoefficient band at a different wheel speed. The adhesion coefficientsare shown as being different with locomotive speed. At a tractive effortbelow the adhesion coefficient band, there is typically no wheel slip.At a tractive effort above the adhesion coefficient band, wheel slip isin an uncontrolled or runaway condition which is characterized in themotoring mode by one or more spinning wheel sets and in the braking modeby one or more skidding wheel sets. Within the adhesion coefficientband, wheel slip is within the region of friction creep where wheel slipis controllable and where some wheels may slip (especially the leadingwheels) and some may not. Maximum tractive or braking effort is obtainedif each powered wheel of the vehicle is rotating at such an angularvelocity that its actual peripheral speed is slightly higher (motoring)or slightly lower (braking) than the true locomotive speed. Thedifference between wheel speed and true speed may be referred to as slipspeed or creep. There is a value of slip speed at which optimum tractiveor braking effort occurs which depends on locomotive speed, rail, gradeand environmental conditions. As long as optimum slip speed is notexceeded, the locomotive will operate in a stable microslip or creepmode. The flexibility of individually controlling power to the tractionmotors allows more precise control and permits all the driving wheels tooperate near the optimum slip speed under all conditions.

A number of variations and modifications of the invention can be used.It would be possible to provide for some features of the inventionwithout providing others. For example in one alternative embodiment,wheel slip can be detected and terminated by slightly decreasing powerto the slipping wheel without measuring an adhesion coefficient andwithout predicting or preempting future occurrences of wheel slip. Inanother alternative embodiment, wheel skid can be detected andterminated by slightly increasing power to the skidding wheel withoutmeasuring an adhesion coefficient and without predicting or preemptingfuture occurrences of wheel skid.

The present invention, in various embodiments, includes components,methods, processes, systems and/or apparatus substantially as depictedand described herein, including various embodiments, subcombinations,and subsets thereof. Those of skill in the art will understand how tomake and use the present invention after understanding the presentdisclosure. The present invention, in various embodiments, includesproviding devices and processes in the absence of items not depictedand/or described herein or in various embodiments hereof, including inthe absence of such items as may have been used in previous devices orprocesses, e.g., for improving performance, achieving ease and\orreducing cost of implementation.

The foregoing discussion of the invention has been presented forpurposes of illustration and description. The foregoing is not intendedto limit the invention to the form or forms disclosed herein. In theforegoing Detailed Description for example, various features of theinvention are grouped together in one or more embodiments for thepurpose of streamlining the disclosure. This method of disclosure is notto be interpreted as reflecting an intention that the claimed inventionrequires more features than are expressly recited in each claim. Rather,as the following claims reflect, inventive aspects lie in less than allfeatures of a single foregoing disclosed embodiment. Thus, the followingclaims are hereby incorporated into this Detailed Description, with eachclaim standing on its own as a separate preferred embodiment of theinvention.

Moreover, though the description of the invention has includeddescription of one or more embodiments and certain variations andmodifications, other variations and modifications are within the scopeof the invention, e.g., as may be within the skill and knowledge ofthose in the art, after understanding the present disclosure. It isintended to obtain rights which include alternative embodiments to theextent permitted, including alternate, interchangeable and/or equivalentstructures, functions, ranges or steps to those claimed, whether or notsuch alternate, interchangeable and/or equivalent structures, functions,ranges or steps are disclosed herein, and without intending to publiclydedicate any patentable subject matter.

1. A method, comprising: (a) in a locomotive comprising a plurality oftraction motors, each driving a plurality of wheels, braking at leastone wheel driven by at least one traction motor (b) determining that theat least one braking wheel is skidding and that the wheels driven by theother traction motors are not skidding; and (c) increasing a power levelapplied to the traction motor driving the at least one skidding wheelwithout increasing the power level applied to the other traction motors.2. The method of claim 1, wherein the power is supplied to the firsttraction motor, wherein at least one of the amplitude, pulse width orfrequency of the power waveform is incrementally increased in theincreasing step (c), and wherein, after each incremental power increase,the incrementally increased power is maintained for a predetermined timeinterval to determine whether wheel skid has stopped as a result of therespective incremental power increase.
 3. The method of claim 1, furthercomprising: (d) determining an operating characteristic in effect at aselected point before and/or during the occurrence of the wheel skid;and (e) using the operating characteristic to predict a later possibleoccurrence of wheel skid.
 4. The method of claim 3, wherein thedetermining step (d) comprises the substep: detecting the operatingcharacteristic of each of the plurality of traction motors and/or atleast one wheel driven by each of the plurality of traction motors; andwherein the operating characteristic is at least one of (i) an armaturevoltage of the corresponding traction motor,(ii) a rotational speed ofone wheel driven by the corresponding traction motor, (iii) a rotationalspeed of the corresponding traction motor, (iv) a current and/or acurrent derivative history of the corresponding traction motor, and (v)a commutator signature in the current of the corresponding tractionmotor.
 5. The method of claim 4, wherein the determining step (d)comprises the at least one of the following substeps: (i) detecting anabrupt decrease to zero of the armature voltage of an individualtraction motor, (ii) detecting an abrupt decrease to zero in therevolutions-per-minute (rpms) of an individual an individual tractionmotor, (iii) detecting an abrupt decrease to zero in therevolutions-per-minute (rpms) of an individual wheel or axle, (iv)detecting an abrupt increase in the traction motor current or currenttime derivative, (v) detecting the disappearance of commutator noise inthe traction motor current, and/or (vi) determining when a wheel speedhas stopped relative to the true ground speed of the locomotive.
 6. Themethod of claim 3, wherein a sensor independently monitors each of thetraction motors and wherein a later possible occurrence of wheel skid isdeemed to exist, the operating characteristic of a first traction motorhas a predetermined relationship with an operating characteristicsetpoint.
 7. The method of claim 3, wherein the determining step (d)comprises: comparing a detected operating characteristic detected foreach of the traction motors to the operating characteristic setpoint andwherein, when the detected operating characteristic has thepredetermined relationship with the operating characteristic setpoint,at least one wheel of a corresponding traction motor is determined to beexperiencing wheel skid.
 8. The locomotive of claim 7, wherein theoperating characteristic is an adhesion coefficient, wherein eachtraction motor has a respective adhesion coefficient characterizing theonset of wheel skid and wherein at least two traction motors havedifferent adhesion coefficients.
 9. A computer readable mediumcomprising instructions to perform the steps of claim
 1. 10. A logiccircuit operable to perform the steps of claim
 1. 11. A locomotive,comprising: a plurality of traction motors, each of the plurality oftraction motors being independently coupled to and driving at least onewheel; a plurality of brakes, at least one of which is operativelyengaged with at least one wheel; and a controller operable (a) to brakeat least one wheel driven by at least one traction motor; (b) determinethat the at least one braking wheel is skidding and that the wheelsdriven by the other traction motors are not skidding; and (c) increase apower level applied to the traction motor driving the at least oneskidding wheel without increasing the power level applied to the othertraction motors.
 12. The locomotive of claim 11, further comprising: aprime energy source; an energy conversion device, in communication withthe prime energy source, to convert the energy output by the primeenergy source into electricity; an energy storage device, incommunication with the energy conversion device and the plurality oftraction motors, to receive and store direct current electricity; aplurality of power control circuits corresponding to the plurality oftraction motors.
 13. The locomotive of claim 11, wherein the power issupplied to the first traction motor in a power waveform, wherein atleast one of the amplitude, pulse width or frequency of the waveform isincrementally increased in the increasing operation, and wherein, aftereach incremental power increase, the controller is operable to maintainthe incrementally increased power for a predetermined time interval todetermine whether wheel skid has stopped as a result of the respectiveincremental power increase.
 14. The locomotive of claim 11 furthercomprising: a processor operable to determine that an operatingcharacteristic in effect at a selected point before and/or during theoccurrence of the wheel skid; and use the operating characteristic topredict a later possible occurrence of wheel skid.
 15. The locomotive ofclaim 14, wherein the processor is operable to detect the operatingcharacteristic of each of the plurality of traction motors and/or atleast one wheel driven by each of the plurality of traction motors; andwherein the operating characteristic is at least one of (i) an armaturevoltage of the corresponding traction motor,(ii) a rotational speed ofone wheel driven by the corresponding traction motor, (iii) a rotationalspeed of the corresponding traction motor, (iv) a current and/or acurrent derivative history of the corresponding traction motor, and (v)a commutator signature in the current of the corresponding tractionmotor.
 16. The locomotive of claim 15, wherein the processor is operableto (i) detect an abrupt decrease to zero of the armature voltage of anindividual traction motor, (ii) detect an abrupt decrease to zero in therevolutions-per-minute (rpms) of an individual an individual tractionmotor, (iii) detect an abrupt decrease to zero in therevolutions-per-minute (rpms) of an individual wheel or axle,(iv) detectan abrupt increase in the traction motor current or current timederivative, (v) detect the disappearance of commutator noise in thetraction motor current, and/or (vi) determine when a wheel speed hasstopped relative to the true ground speed of the locomotive.
 17. Thelocomotive of claim 14, wherein a sensor independently monitors each ofthe traction motors and wherein a later possible occurrence of wheelskid is deemed to exist the operating characteristic of a first tractionmotor has a predetermined relationship with an operating characteristicsetpoint.
 18. The locomotive of claim 14, wherein the processor isoperable to compare a detected operating characteristic detected foreach of the traction motors to the operating characteristic setpoint andwherein, when the detected operating characteristic has thepredetermined relationship with the operating characteristic setpoint,at least one wheel of a corresponding traction motor is determined to beexperiencing wheel skid.
 19. The locomotive of claim 18, wherein theoperating characteristic is an adhesion coefficient, wherein eachtraction motor has a respective adhesion coefficient characterizing theonset of wheel skid and wherein at least two traction motors havedifferent adhesion coefficients.
 20. A method, comprising: (a) in alocomotive comprising a plurality of traction motors, each driving aplurality of wheels, braking at least one wheel driven by at least onetraction motor; and (b) during braking, continuing to apply a voltage toeach of the traction motors, the applied voltage being less than orequal to a back electromotive force of each traction motor, whereby,when the at least one wheel skids and the back electromotive forcedisappears, the corresponding traction motor, in response to the appliedvoltage, thereupon applies a torque to the at least one wheel therebyresisting continued skidding of the at least one wheel.
 21. The methodof claim 20, further comprising: when the applied torque is notsufficient to overcome skidding of the at least one wheel, increasing apower level applied to the corresponding traction motor driving the atleast one skidding wheel.
 22. The method of claim 21, wherein the poweris increased without increasing the power level applied to the othertraction motors.